1. Field of the Invention
This invention relates to a driving force control system for a front-and-rear wheel drive vehicle of a type which drives front wheels and rear wheels independently by first and second prime movers, respectively.
2. Description of the Prior Art
Conventionally, a driving force control system of the above-mentioned kind was proposed e.g. by Japanese Patent Publication (Kokai) No. 2000-79833. The front-and-rear wheel drive vehicle has front wheels thereof driven by an engine and rear wheels thereof driven by a motor. A battery as a drive source for the motor can be charged by recovering running energy of the vehicle. Further, in the driving force control system, on condition that the remaining charge of the battery is equal to or smaller than a predetermined value, and when a driver""s operation for accelerating the vehicle has been carried out e.g. by stepping on an accelerator pedal, the gear ratio of a continuously variable transmission is changed toward a higher gear ratio side by a predetermined amount. As a result, a surplus torque is generated, and the battery is charged by a regenerative braking torque corresponding to the surplus torque. This battery charge is continued until the remaining charge of the battery reaches the predetermined value. According to the proposed driving force control system, the charging of the battery is carried out not only during deceleration of the vehicle, but also at other times as described above, whereby the remaining charge of the battery is prevented from becoming insufficient.
However, in the above conventional driving force control system, when the remaining charge of the battery is equal to or smaller than the predetermined value, recharge traveling of the vehicle is carried out unconditionally in response to operation of the accelerator pedal so as to preserve a required amount of remaining charge. This means that despite the fact that a high driving force is desired by the driver who is demanding acceleration of the vehicle, part of the output of the engine is consumed as a regenerative braking torque for charging the battery, which increases load on the engine, thereby causing degradation of fuel economy. Further, the change gear ratio of the continuously variable transmission is changed toward the higher gear ratio side due to the recharge traveling, which can cause a downshift unexpected by the driver. This downshift degrades drivability while giving an uncomfortable feeling to the driver. In addition, since the charging of the battery is simply carried out until the remaining charge reaches the predetermined value, in some operating condition of the vehicle, sudden generation and loss of the regenerative braking torque can occur at the start and end of the recharge traveling, causing torque changes. This also leads to degradation of drivability.
In the above situation, the rear wheels receive the regenerative braking torque acting in a decelerating direction, whereas the front wheels receive a drive torque from the engine, which acts in an accelerating direction. As a result, the front wheels and the rear wheels are driven in respective opposite directions, and at the same time, the torque acting on the front wheels is increased by the amount of the regenerative braking torque acting on the rear wheels. Therefore, e.g. during running of the vehicle on a low friction road, the difference in rotational speed between the front wheels and the rear wheels is increased, whereby slippage of the rear wheels can be also increased, throwing lateral forces acting on the respective front-and-rear wheels out of balance, which makes the behavior of the vehicle unstable.
It is a first object of the invention to provide a driving force control system for a front-and-rear wheel drive vehicle, which is capable of charging a battery properly and easily depending on a traveling condition of the vehicle and in response to the intention of the driver, thereby improving fuel economy and drivability of the vehicle.
It is a second object of the invention to provide a driving force control system for a front-and-rear wheel drive vehicle, which is capable of limiting the difference in rotational speed between front wheels and rear wheels, which is produced during a recharge mode for charging a drive source of a second prime mover with driving energy, within a proper range dependent on a state of a road surface and a traveling condition of the vehicle, thereby ensuring traveling stability of the vehicle.
To attain the above first and second objects, the present invention provides a driving force control system for a front-and-rear wheel drive vehicle that is driven while switching between a drive mode for driving first drive wheels as one pair of respective pairs of front drive wheels and rear drive wheels by a first prime mover and driving second drive wheels as another pair of the respective pairs by a second prime mover, and a recharge mode for recovering running energy of the vehicle to thereby charge a drive source of the second prime mover with the running energy recovered as driving energy.
The driving force control system according to the invention is characterized by comprising:
parameter detecting means for detecting a parameter indicative of a driving condition of the vehicle;
traveling condition-determining means for determining a traveling condition of the vehicle based on the parameter;
driving energy charge execution-determining means for determining, based on the traveling condition of the vehicle and a state of charge of the driving energy in the drive source, whether or not charging of the drive source with the driving energy is to be executed; and
energy charge-executing means for executing the charging of the drive source with the driving energy.
According to the driving force control system of the invention, it is determined, based on the traveling condition of the vehicle and the state of charge of the driving energy in the drive source, whether or not charging of the drive source with the driving energy is to be executed. This makes it possible to improve performance of the vehicle, that is, enhance fuel economy and drivability of the vehicle can be enhanced, and ensure traveling stability of the same.
Preferably, to attain the first object, the parameter detecting means includes accelerator pedal opening-detecting means for detecting an accelerator pedal opening, and vehicle speed-detecting means for detecting a vehicle speed, the traveling condition-determining determining means including desired driving force-calculating means for calculating a desired driving force based on the detected accelerator pedal opening and the detected vehicle speed, reference driving force-storing means for storing a reference driving force which is set, based on a driving force of the vehicle required for cruising, to a larger value than the driving force, and driving force comparison means for performing a comparison between the desired driving force and the reference driving force, and the driving energy charge execution-determining means including driving energy charge-permitting means for permitting the charging of the drive source with the driving energy when the driving force comparison means determines that the desired driving force is smaller than the reference driving force.
According to this driving force control system, the reference driving force is set, based on the driving force of the vehicle required for the cruising, to a predetermined value which is larger than the driving force, and when it is determined that the desired driving force is smaller than the reference driving force, the charging of the drive source of the second prime mover with the driving energy by recovery of the running energy of the vehicle is permitted, whereby execution of the recharge mode is started. Thus, in a cruising condition of the vehicle, the drive source of the second prime mover is charged with the driving energy when the desired driving force becomes smaller than the reference driving force. In this case, the vehicle is cruising or running at a constant speed, so that the desired driving force for the whole vehicle is small, and hence an increase in load on the first prime mover due to recovery of the running energy is small. On the other hand, in an accelerating state of the vehicle e.g. with its accelerator pedal being stepped on, the desired driving force increases and exceeds the reference driving force, so that the charging of the drive source with the driving energy is inhibited. Thus, it is possible to charge the drive source of the second prime mover with the driving energy only when there is no driver""s demand for acceleration, without applying extra load on the drive source of the first prime mover, so that fuel economy and drivability of the vehicle can be further improved than the prior art.
Further, the reference driving force serves not only as a criterion for determining whether or not the vehicle is in a cruising condition suitable for the charging of the driving energy, but also as a criterion for executing the charging of the driving energy, so that simply by presetting the reference driving force and storing the same in the system, it is possible to perform determination as to the cruising condition of the vehicle and carry out charging of driving energy, easily and properly.
More preferably, the reference driving force is a driving force of the vehicle required for cruising on a road having a predetermined upward slope.
According to this preferred embodiment, when the vehicle is cruising on a road having a smaller slope than the predetermined upward slope, the desired driving force is always smaller than the reference driving force, and this makes it possible to ensure a traveling region for executing the charging of driving energy, whereby the charging of the drive source with the driving energy can be carried out reliably. Further, the reference driving force can be set easily by calculation.
More preferably, the driving force control system further comprises stored driving energy amount-detecting means for detecting a stored amount of the driving energy in the drive source, and reference driving force-correcting means for correcting the reference driving force according to the detected stored amount of the driving energy.
According to this preferred embodiment, since the reference driving force is corrected according to an actually stored amount of driving energy, it is possible to charge the drive source with the driving energy efficiently and properly in accordance with a degree of requirement.
More preferably, the driving force control system further comprises driving energy charging-rate setting means for setting a charging rate at which the drive source is charged with the driving energy, according to a difference between the desired driving force and the reference driving force.
According to this preferred embodiment, it is possible to properly set the charging rate at which the drive source is charged with the driving energy according to the difference between the desired driving force and the reference driving force as well as to start and terminate recharge traveling in a state of a regenerative braking torque (amount of drag torque) being very small. As a result, sudden generation and loss of regenerative braking torque at the start and end of recharge traveling is prevented, which makes it possible to execute the recharge traveling without giving any uncomfortable feeling to the driver.
Preferably, to attain the above second object, the parameter detecting means includes first drive wheel speed-detecting means for detecting a speed of the first drive wheels, and second drive wheel speed-detecting means for detecting a speed of the second drive wheels, the traveling condition-determining means including drive wheel speed comparison means for performing a comparison between the detected speed of the first drive wheels and the detected speed of the second drive wheels, the driving force control system further comprising driving energy charging rate-limiting means for limiting a charging rate at which the drive source is charged with the driving energy, when the drive wheel speed comparison means determines, during the recharge mode, that the speed of the second drive wheels has become lower than the speed of the first drive wheels by more than a predetermined value.
According to this driving force control system, the speed of the first drive wheels driven by the first prime mover and the speed of the second drive wheels driven by the second prime mover are detected, and the detected speeds of the respective first and second drive wheels are compared with each other by the drive wheel speed comparison means. Then, during the recharge mode for recovering running energy of the vehicle to charge the drive source of the second prime mover with the recovered running energy, when the speed of the second drive wheels is determined to have become lower than the speed of the first drive wheels by more than a predetermined value, the charging rate of driving energy is limited by the driving energy charging rate-limiting means.
When the speed of the second drive wheels becomes lower than the speed of the first drive wheels by some margin due to regenerative braking by the second drive wheels during the recharge mode, the charging rate at which the drive source of the second prime mover is charged with the driving energy, i.e. the regenerative braking torque of the second drive wheels, is limited, so that the difference in speed between the first drive wheels and the second drive wheels, i.e. the slippage of the second drive wheels, can be caused to remain within a proper range. As a result, even when the vehicle is traveling on a low-friction road or the like, it is possible to positively reduce slippage of the second drive wheels during the recharge mode, thereby ensuring traveling stability of the vehicle.
More preferably, the driving energy charging rate-limiting means limits the charging rate of the driving energy in dependence on a degree of decrease in the speed of the second drive wheels relative to the speed of the first drive wheels.
According to this preferred embodiment, since the charging rate of the driving energy is limited according to the degree of decrease in the speed of the second drive wheels relative to the speed of the first drive wheels, it is possible to more properly control the regenerative braking torque of the second drive wheels depending on the magnitude of an actual frictional resistance of the road surface on which the vehicle is running, to thereby ensure more excellent traveling stability of the vehicle.
More preferably, the recharge mode includes a decelerating traveling-time recharge mode to be executed during decelerating traveling of the vehicle, and a non-decelerating traveling-time recharge mode to be executed during non-decelerating traveling of the vehicle, the predetermined value for use in the non-decelerating traveling-time recharge mode being set to a smaller value than the predetermined value for use in the decelerating traveling-time recharge mode.
In the non-decelerating traveling-time recharge mode, the second drive wheels receive regenerative braking torque acting in a decelerating direction, whereas the first drive wheels receive drive torque acting in an accelerating direction. As a result, the first and second drive wheels are driven in respective opposite directions, which is apt to give an uncomfortable feeling to the driver. On the other hand, in the decelerating traveling-time recharge mode, since the first and second drive wheels are driven in the same direction, the driver never feels such uncomfortableness. Therefore, according to this preferred embodiment, since the predetermined value for use in determining whether or not limitation should be imposed on the charging rate of driving energy is set as above, the limitation on the charging rate is more promptly started in the non-decelerating traveling-time recharge mode than in the decelerating traveling-time recharge mode. This makes it is possible to maintain the drivability of the vehicle properly in accordance with a traveling condition of the vehicle.
The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.